Solvent based adhesives have been used extensively to join thermoplastic pipe and fittings for numerous years. These solvent based adhesives provide for a convenient way to join thermoplastic materials relatively easily and quickly. Often, thermoplastic pipe and fittings joined in this manner can even be tested the same day.
Generally, the solvent based adhesives comprise a mixture of solvents as well as resin and other additives such as thixotropic agents. The solvent based adhesive dissolves the surface layer of the thermoplastic material to which it is applied, causing it to swell. The resin in the adhesive solution accelerates the setting of the two materials to be joined, fills any voids, as well as reduces the internal stresses. The adhesive cures by evaporation. The primary solvents used in conventional solvent based adhesives include tetrahydrofuran, methyl ethyl ketone and cyclohexanone. These solvents are very volatile and consequently adhesives made therefrom have VOC levels in the range of 750 to 850 g/l as measured by the South Coast Air Quality Management District (SCAQMD) 316A.
Furthermore, prior to the application of these conventional solvent based adhesives, the thermoplastic material desirably is prepared with either a primer such as tetrahydrofuran or a cleaner such as acetone, in order to minimize adhesive failure. In this cleaning process even more volatile organic compounds are released into the atmosphere. In addition, since these conventional solvent based adhesives are largely formed from solvents, the solvent tends to spread to a large area and drip during their application to the thermoplastic materials, causing additional volatization. Moreover, conventional solvent based adhesives and/or primers for adhesives generally have low flash points. The low flash point requires special precautions in the handling and packaging to avoid combustion of these adhesives and/or primers. Evaporation of solvents from solvent based adhesives provides for an air pollution problem.
Due to the environmental awareness occurring today, laws and regulations are being enacted to limit the amount of VOC levels in all materials, in particular solvent based adhesives. In California, for example, the South Coast Air Quality Management District (SCAQMD) has set regulations limiting the VOC levels of materials used to join thermoplastic materials. For example, pursuant to Rule 1168 of SCAQMD, the VOC limits for CPVC and/or polyvinyl chloride (PVC) solvent based adhesives, effective Jan. 1, 1994 were 450 grams/liter as measured by SCAQMD 316A. The VOC limits for acrylonitrile styrene butadiene (ABS) solvent based adhesives were 350 grams/liter as of Jan. 1, 1994 as measured by SCAQMD 316A. Further legislation has reduced these limits even further. Future proposed regulations, the SCAQMD VOC limits for CPVC and PVC solvent based adhesives will be 250 grams/liter whereas the SCAQMD VOC limits for ABS solvent based adhesives will remain at 350 grams/liter.
Several adhesives have been formulated which contain lower VOC levels than the conventional solvent based systems. The VOC levels of conventional solvent based adhesive systems was generally about 650 grams/liter as measured by SCAQMD 316A prior to 1994. For example, Australian Patent Application 86750/91 discloses an adhesive comprising more than 80 weight percent of n-methyl-2-pyrrolidone, more than 0.25 weight percent of a viscosity modifier and more than 10 weight percent of a vinyl based polymer. The viscosity modifier can be silica, a thickening agent or a thixotropic agent. Similarly, U.S. Pat. No. 4,675,354 discloses a glue solution which comprises a solution of a water insoluble synthetic organic polymer in a solvent such as N-methyl-2-pyrrolidone. This glue solution may be used at tropical temperatures without problems arising from solvent vapors and fire risks.
Also, U.S. Pat. No. 4,687,798 discloses a solvent cement used for joining water insoluble polymers. The solvent cement comprises about 10-15 weight percent of a water insoluble polymer and a solvent. The solvent comprises ethyl acetate and N-methyl-2-pyrrolidone. The ethyl acetate ranges from about 3 percent to about 50 percent by weight of the solvent with the balance being N-methyl-2-pyrrolidone.
In addition, European Patent Application 0 547 593 A1 discloses a low VOC adhesive composition. The composition of this European Patent Application comprises a mixture of from 5 weight percent to about 60 weight percent of at least one water insoluble polymer, from about 1 weight percent to about 30 weight percent of inorganic or synthetic resinous hollow microspheres and from about 20 weight percent to about 70 weight percent of at least one volatile organic liquid which is a solvent for the water insoluble polymer.
U.S. Pat. No. 5,470,894 to Patel et.al., provides for an additional example of a low VOC solvent based adhesive. The low VOC solvent based adhesive in this patent is used to join CPVC pipes. The adhesive comprises a high vapor pressure solvent comprising from about 15 to about 35 weight percent of tetrahydrofuran and 0 to about 30 weight percent of methyl ethyl ketone; a low vapor pressure solvent comprising about 20 to about 45 weight percent cyclohexanone, 0 to about 30 weight percent of N-methyl pyrrolidone and from 0 to 10 weight percent of dibasic esters. Patel, et. al. state that the VOC level of their adhesive is at or below 450 grams/liter, while the adhesive meets or exceeds the required performance standards such as hydrostatic burst strength and hydrostatic sustained pressure tests.
Nonetheless, there is still an environmental concern with using any one of the above enumerated adhesives. There are however, alternatives to solvent based adhesives. These are mechanical, reactive, or thermal systems. Mechanical joining systems are generally very expensive to use. Examples of mechanical joining systems include Acorn Fittings from Hepworth Building Products; PolyGrip Fittings from Philmac Corporation and Uncopper Fittings from Genova. Thermal systems are unpredictable due to the difficulty in consistently producing adequate pipe/fitting unions. Examples of thermal systems include hot melt glues available from the Minnesota, Mining and Manufacturing Company. These thermal systems are difficult to apply and perform less consistently than solvent based adhesives. An example of a reactive system includes epoxy. Epoxy is available from the Noble Corporation under the tradename Copper Bond. Other examples of an epoxy include General Purpose Urethane, High Shear Strength Urethane and All Purpose Epoxy, all available from the Hardman Corporation. However, these reactive systems are problematic because they have long cure times and poor green strength. Their efficacy is also temperature dependent; at low temperatures epoxy materials have very long cure times. Furthermore, there may be by products of the chemical reactions which may be detrimental to the strength of the pipe. Even though these alternatives exist, they are cost prohibitive, time consuming and cumbersome.
Despite some of the air quality problems, there are benefits to continuing to use the solvent based adhesives to join thermoplastic materials. First, solvent based adhesives are easy to use and many workers have years of experience using these types of adhesive systems. Second, there are low production costs in making the solvent based adhesives as well as long term durability once the adhesives are used to join the two thermoplastic materials. Further, the solvent based adhesives can be used on location to join the two thermoplastic materials together without any additional equipment. Fourth, the solvent based adhesive system cures pretty rapidly, allowing for testing. In addition, one technique can be used to apply the solvent system for all sizes of pipe. Generally, the solvent based adhesive system can be applied to the joint at any temperature in the range of 0.degree. to 120.degree. F., if the solvent based adhesive system meets the Underwriter's Laboratories Test 1821. Also, the solvent based adhesive systems do not rely upon a chemical reaction for their efficacy. Moreover, the solvent based adhesive system can possibly be stored long term at ambient temperatures. Therefore, overall the solvent based adhesive systems are generally practical and economical.
Thus, there currently exists a need for a low VOC solvent based adhesive which has adequate shelf and storage life. Furthermore, there exists a need for a low VOC solvent based adhesive that meets the required performance criteria necessary to join two thermoplastic materials together. Additionally, there exists a need for a low VOC solvent based system with higher flash points than conventional solvent based adhesives and/or primers for adhesives.